The whole-cell pertussis vaccine currently in use in the U.S. provides insufficient efficacy (80%), limited duration of immunity, and worrisome side effects. We propose to conduct an efficacy trial of a new acellular pertussis (Ptx) vaccine in Sweden (Goteborg), where vaccination with whole-cell vaccine was discontinued in 1979, and where whooping cough is now endemic. We are now conducting a phase 2 study with 120 infants age 7-10 weeks at enrollment, who will be immunized at 3 separate times at two month intervals. A blood sample will be collected from each infant at visit 1 and 3, and 4-6 weeks after the last injection. A blood sample from the mother will also be collected at the first visit. Information will be collected from parents about any reactions the child may have to the vaccine, and any other health events in the follow-up period. Sera will be analyzed for antitoxin and IgG PT levels. Active surveillance for pertussis cases in the Goteborg area is now in effect. If the results of this pilot study are considered satisfactory by the appropriate review committees, we propose to carry out an efficacy trial of the Ptx vaccine. We will try to demonstrate, through a double-masked, controlled, randomized trial, that infants receiving a 3-dose course of Ptx vaccine will have a lower incidence of pertussis than infants who do not receive the vaccine. We will, as in the phase 2 study, immunize infants age 7-10 weeks at enrollment, for 3 separate injections at two month intervals. Bloods will be collected from infants and mothers, as before, and will also be analyzed for antitoxin and IgG PT levels. Active surveillance for reactions and disease will, of course, also be instituted.